The present invention relates generally to power generation systems in which energy is extracted from a hot pressurized gas, and, more specifically, to improving efficiency thereof.
The present invention further pertains to methods and systems for producing mechanical energy or electric power in which a combustion turbine is used for conversion of the chemical energy in a fuel. This invention also relates to methods and systems for increasing the performance for gas turbines, prime movers, internal combustion engines, afterburning Ericsson cycle engines, Rankine cycles, combined cycles etc. Additionally, it pertains to new methods and systems for cooling the inlet air of gas turbines and simultaneously humidifying this air prior to combustion to increase power output and combustion efficiencies.
It is known that the operation of a turbine is thermodynamically described by the Brayton cycle, which is basically a collection of repetitive sequential energy transfer processes. A gas turbine engine generally drives the compressor stage, but it is more appropriate to think of it as an axial compressor, which is like a rotating fan of some kind. In this stage, the working gaseous fluid (air) is compressed in a series of compressor blades turned by a turbine.
The burning of fuel in the combustion section then drives the hot gases through the turbine blades and powers them. An interconnecting shaft from the compressor section through the turbine section extends out to drive a generator or pump or other mechanical rotating device.
This compression process is adiabatic, and it raises both the temperature and pressure of the compressed air. After the air is compressed, fuel is added and energy is transferred to a high temperature, high pressure gas stage because fuel burns, or because there is some kind of heat exchanger fueled by a hot source. Its temperature then rises still further, but the engine is arranged so that at the same time the volume of the gas is allowed to increase; overall, therefore, the gas remains at constant pressure.
The hot, expanded gas then enters the turbine stage where there is an adiabatic expansion of the gas against the turbine blades. This cools the gas and extracts its energy as work resulting from the transformation of the incoherent thermal motion of the hot gas into the coherent rotational motion of the blades of the turbine.
The last stage then lowers the gas temperature at constant volume via dumping heat into a sink in order to complete the cycle to achieve a viable engine. The technical difficulty of making this cycle practical requires hot and cold devices to be separated so that the turbine is kept at a high temperature while it is running.
As a thermodynamic process, it is well known that the performance of a gas turbine or internal combustion engine can be increased by cooling the air inlet, increasing its density and the air mass flowing into the engine, the compressor blades or cylinders. As the ambient temperature increases, the demand on electric utilities usually increases from air conditioning equipment while the capacity of combustion turbine decreases.
A means to overcome the high ambient temperature and decreased generation capacity is to cool the compressor inlet air. The performance of a typical industrial combustion turbine is shown in the book by William E. Stewart, “Design guide: combustion turbine inlet air cooling systems”, 1999, American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., Atlanta; p. 8, FIG. 9, where the capacity of the turbine varies nearly linearly with the inlet air temperature. For example, on a 100° F. (37.8° C.) day, the capacity of a turbine at the Lincoln, Nebr. site is approximately 73 MW. Cooling the combustion inlet air to 40° F. (4.4° C.) increases the turbine output to approximately 90 MW, an increase of 17 MW, or 23%.
Some turbines may experience a 1% decrease in heat rate for every 5.5° F. (3.1° C.) of air cooling. Turbines operating at lower inlet air temperature may also have extended life and reduced combustion chamber overhauls and other maintenance.
Typically, cooling the combustion inlet air is accomplished via an evaporative cooler associated with the air inlet stream entering the compressor. This choice is due to the low capital cost to install an evaporative cooler in comparison to that of a refrigerated air system. Existing evaporative cooling using treated water is relatively inexpensive, but the decrease in inlet air temperature is limited to approaching the ambient saturated wet bulb temperature. Thereby the turbine performance increase when associated with an existing evaporative cooler is limited to the ambient wet bulb temperature of the region.
As such, evaporative cooling is most advantageous in drier climates where the required water supply is not limited. In addition, conventional evaporative coolers are wet systems with high maintenance problems associated with scaling. Such cooling has been attempted with direct evaporative coolers with cool air by adding moisture. However, as entrained moisture evaporates, dissolved salts in the water precipitate out forming entrained dust, which can erode the gas turbine parts. They are also dependent on water availability and price.
The invention described in U.S. Pat. No. 4,418,527, proposes a system for cooling and humidifying air entering a gas turbine. The air is first cooled on the dry side of an indirect evaporative heat exchanger and then moistened by a fine water spray of distilled water. This distilled water is obtained from a boiler, which is heated with excess heat from the gas turbine and condensed by the moist cool air, leaving the wet side of the indirect evaporative heat exchanger. This invention has two serious disadvantages. The temperature of the cooled air entering in the gas turbine is limited to the ambient wet bulb temperature as this air is humidified by water; consequently its density increase is small.
The design must ensure that there is no water carryover from evaporative media or cooling coils, which may have a detrimental effect on turbine blade life. Also if there is more humidity in air, there is less density and consequently less air mass flowing into the gas turbine compressor.
All known and available evaporative cooling methods and designs for cooling the inlet air of a gas turbine have one common disadvantage: the minimum temperature that may be reached is the wet bulb temperature of outside air. It cannot guarantee the efficient rejection of heat from an inlet air stream.
U.S. Pat. No. 5,790,972 describes a method and apparatus for cooling the inlet air of a gas turbine, which increases the performance of a gas turbine by cooling the air inlet below wet bulb temperature to densify the air mass flowing into the gas turbine compressor blades. But this system is very complicated and expensive. It contains three cooling coils, three water/brine chillers, three compressors, pumps, cooling tower or evaporative condenser and etc. This equipment also needs expenditure of energy for operation, and reduces the reliability of operation of the system for cooling the air inlet.
Therefore, although an existing evaporative cooler is workable and economical, particularly in low humidity areas, additional performance can be achieved by a source of lower temperature of cooling the inlet air for gas turbine without humidifying it. This is especially the case where additional gas turbine performance is achieved at temperatures below wet bulb temperature.
When a working fluid is used in an engine to produce mechanical energy or electrical power from the chemical energy contained in a fuel, the working fluid is pressurized and, following combustion of the fuel, the energy thus released from the fuel is absorbed into the working fluid as heat. The working fluid with the absorbed energy is then expanded to produce mechanical energy, which may in turn be used to drive a generator to produce electrical power.
Unconverted energy is rejected in the exhaust in the form of heat, only a portion of which may be recovered and utilized. The efficiency of the engine is at a maximum when the temperature of the working fluid entering the expansion stage is also at a maximum.
Expansion in the turbine produces the mechanical energy, and the turbine exhaust carries off the unconverted heat. The efficiency of the combustion turbine is at a maximum when the combustion temperature itself is at a maximum, and this occurs when the fuel is burned in the presence of pressurized air under stoichiometric conditions, i.e., enough air is present for complete combustion, but without any excess.
When fuel oil is burned with air under stoichiometric conditions, however, the resulting temperature is approximately 4000° F., which is in excess of the metallurgical limits of the turbine. As a result, it is necessary to utilize a large excess of air in the combustion step, which acts as a thermal diluent and reduces the temperature of the combustion products to approximately 2000° F. The necessity to use a large excess of air under pressure in turn creates a large parasitic load on the system, because compression of the air requires mechanical energy and thus reduces the net power produced from the system, as well as reducing the overall efficiency of the system.
Another disadvantage of existing combustion turbine cycles is that the pressurization step requires compression of air. Compression of a gas is very inefficient, since mechanical energy is required, which is the highest form of energy and degrades into thermal energy. The mechanical energy required for air compression can be reduced by utilizing interstage cooling, that is, by cooling the temperature of the compressed air between successive stages of a multiple stage compression process.
However, from an overall cycle efficiency standpoint, interstage cooling can be utilized advantageously if the heat removed from the compressed air in the intercooler can be efficiently recovered and utilized. If the entire heat is simply rejected to the atmosphere, the overall cycle efficiency is actually decreased, since it results in the consumption of more fuel to compensate for the energy lost through the intercooler. Accordingly, rather than simply rejecting the heat, in commercial practice, the high compressor horsepower requirement has been tolerated while containing the heat in the compressed airstream.
Even in light of the foregoing limitations, it is very desirable to use a combination turbine engine, because it is able to operate at the highest temperature of engines that use a working fluid to convert chemical energy in a fuel to mechanical energy. However, due to the high exhaust temperature that is inherent in a combustion turbine engine, the efficiency of the cycle is limited, and as a result, the exhaust from the engine is used as the heat source to operate another engine such as a steam turbine to increase the overall efficiency of utilization of the fuel. Such a system is called a combined cycle system and is widely used in the industry.
Another use for the energy contained in the combustion turbine exhaust is to raise superheated steam which is injected back into the combustion chamber of the combustion turbine, see, e.g., U.S. Pat. No. 3,978,661. Yet another method is to preheat the air leaving the compressor against the engine exhaust and simultaneously use interstage cooling during compression. These systems show higher overall efficiencies with respect to the utilization of the chemical energy contained in a fuel, but as will be explained subsequently herein, are inherently less efficient than the process of the present power system.
A combined cycle cannot take full advantage of air compressor intercooling because the temperature of the heat rejected in the air compressor intercooler is too low to be recovered for efficient use, such as for steam generation. A small portion of this heat may be recovered for boiler feed water preheating as described in U.S. Pat. No. 3,335,565 but this results in more heat being rejected with the stack gases and results in little, if any, net increase in either heat recovery or cycle efficiency.
Recently, direct water injection into the airstream as a means of intercooling has been proposed. However, there are two disadvantages with this. One is that the dew point temperature of the saturated air limits the temperature of air leaving the intercooling step. Also, by the direct injection of water into the air in the intercooler, the added water vapor which serves as a thermal diluent needs to be compressed in the successive stages after the intercooler, which precludes realizing the full advantage of water vapor substitution as a means of saving compression power.
U.S. Pat. No. 2,869,324, describes evaporation of water into the compressed air after preheating both the air and the water. However, this means of evaporation requires a higher temperature level to achieve useful moisture loading of the air because the air and water leave the evaporator in equilibrium with each other. This method of water evaporation is less efficient than the present power system, which can take advantage of air entering the saturator at low temperatures.
The steam cycle has an inherently high irreversibility since the evaporation of water (steam generation) occurs at a constant temperature, whereas the heat release occurs at varying temperatures. With steam generation, a small temperature difference between the heat source and heat absorbing fluid cannot be maintained, and this leads to a high irreversibility in the system and hence a lower efficiency. A combined cycle plant is also expensive since it requires an additional steam turbogenerator, steam drums, surface condenser for condensing steam turbine exhaust, and cooling towers to reject the heat from the surface condenser to the atmosphere.
A steam-injected cycle cannot take full advantage of air compressor intercooling for the same reasons as a combined cycle. Also this cycle involves the generation of steam and hence has the same irreversibility associated with it as described for a combined cycle, although eliminating the steam turbogenerator, surface condenser and cooling towers, and reducing the parasitic load of air compression by displacing some of the air with steam.
This is an improvement over the water-injected cycle described in NASA Report No. TR-981 titled “Theoretical Analysis of Various Thrust-Augmentation cycles for Turbojet Engines”, by B. L. Lundin, 1950, where liquid water is directly injected into the combustion chamber. The injected water displaces some of the diluent air, but there is a tremendous irreversibility associated with this. The evaporation of the liquid water in the combustion chamber uses energy from the fuel at the highest temperature, which results in an overall reduction of efficiency. Also with the water-injected cycle, the heat available from the turbine exhaust still remains to be utilized.
The heat used for generation of steam in a steam-injected cycle is of a much higher quality, i.e., temperature level, than is desirable. For example, typically for a combustion turbine operating at pressure ratio of 11, the steam pressure required for injection should be at least 200 psia. The corresponding saturation temperature of the steam is 382° F. This requires that a heat source be available at much higher temperatures, for example 420° F.
In some cases, steam injection may decrease turbine capacity or increase heat rates and CO emissions. Lower inlet air temperature using a combustion turbine inlet air cooling system reduces oxides of nitrogen (NOx) emissions by lowering the combustion gas temperatures and can possibly eliminate the need for steam/water injection for NOx control.
The intercooled regenerative cycle uses intercooling during the air compression step, and compressed air preheated against the turbine exhaust before the air enters the combustion chamber. The optimum pressure ratio for this cycle is about 6 to 7. The heat released in the intercooler is all lost to atmosphere. Also the temperature of gas leaving the air preheater is around 500° F., and the heat contained in these gases is all wasted. All the thermal diluent is compressed, leading to a large parasitic load, which results in poor overall efficiency for the system.
U.S. Pat. No. 2,186,706 describes the replacement of a portion of the air for combustion with water vapor derived by directly contacting the compressed air with heated water in a humidification operation. The heat required for this humidification operation is supplied by intercoolers in the air compressor. Makeup water for the system picks up additional heat from gas turbine exhaust. The net effect of such a system is a reduction in the parasitic load of air compression and, thus, an increase in cycle efficiency.
Nakamura et al., in U.S. Pat. No. 4,537,023, describes a system similar to that of U.S. Pat. No. 2,186,706, in which an aftercooler is used for the air compressor. The aftercooler reduces the temperature of the water leaving the humidifier, which in turn allows recovery of lower level heat to a greater extent. The decrease in heat-rate resulting from the addition of the aftercooler is approximately 1.4 percent, based on the data presented in the Nakamura et al, patent.
Both systems (see U.S. Pat. Nos. 2,186,706 and 4,537,023) reject heat from the cycle through the stack gases. Rejection of heat is a consequence of the second law of thermodynamics and any power cycle converting heat to power must reject some heat. To improve the cycle efficiency, it is not only important to minimize the quantity of heat being rejected, but also, to minimize the temperature at which the heat is rejected. In both these systems, the quality of heat being rejected is solely set by the stack temperature, which constrains the cycle efficiency.
Accordingly, all known evaporative humidification processes are not effective and have limited humidity corresponding to the wet bulb temperature of outside air.
William Nebgen (see U.S. Pat. No. 3,877,218) describes Brayton Cycle Systems with Refrigerated Intake and Condensed Water Injection, which increases the performance of gas turbine by cooling the inlet air of the compressor with a vapor compression refrigeration system and injecting condensed water from the system into the air before combustion. This system contains an air compressor, a combustion chamber, a gas turbine and also a conventional refrigerating system with refrigerant evaporation and condensation. The disadvantages of this system are big cost and greater consumption of the energy.
The output of gas turbines can be enhanced using low-pressure steam (see U.S. Pat. No. 5,241,816). Pumped water circuit and water contact towers are used to achieve desired gas humidification using such low-pressure steam. But this humidification process only for gases cannot significantly increase power output and combustion efficiencies.
The present power system also relates to methods for increasing the performance not only for gas turbines but also for internal combustion engines, or external combustion engines, for example afterburning Ericsson cycle engines. An example of the internal combustion approach is the “Afterburning Ericsson Cycle Engine” disclosed in U.S. Pat. No. 5,894,729.
The heat engine operates on the afterburning Ericsson cycle, whose principle is heat addition to the cycle by an afterburner in which fuel is burned with the low-pressure air working fluid exhausted by the expander. The resulting combustion gases are used in a countercurrent heat exchanger continually heating the air expanding in the expander. But using this generator it is impossible to realize processes for cooling the inlet air of a compressor and simultaneously humidifying this air prior to combustion to increase their power output and combustion efficiencies.
U.S. Pat. No. 4,829,763 describes producing mechanical energy or electric power from chemical energy contained in a fuel, utilizing a combustion turbine. The compressed air, which is used for combustion of the fuel to drive the turbine, is humidified prior in a multistage saturator to replace some or all of the thermal diluent air with water vapor. Humidification of the compressed air increases the mass flow through the gas turbine and thereby raising the power output. Humidification is effected with the water at a temperature below its boiling point at the operating pressure. The compressed air is cooled prior to humidification by passing in heat exchange relationship with the water used for humidification. This process provides an improvement in thermal efficiency, compared to conventional cycles.
But this improvement is not much, and the known process for producing power has a lot of disadvantages:                1. The known system doesn't provide the means for humidifying the compressed air in a thermodynamically efficient manner and consequently cannot guarantee a high level of moisture for this air, using heat from stack gas from a turbine.        2. The known system doesn't provide the means for cooling the inlet air to the compressor of a gas turbine (especially below the wet bulb temperature as it approaches the dew point temperature without adding humidity) to increase their power output and combustion efficiencies.        3. Process of rejecting heat from compressed air during intercooling and just prior to humidification is not efficient, leading to a large parasitic load, which results in poor overall efficiency for the system.        4. The known system which realizes this process for producing power is very complicated and expensive. It contains a lot of pumps and apparatuses, where heat and mass exchange processes are realized but with poor efficiency.        5. The known system cannot provide simultaneous cooling process for the inlet air before an air compressor, and humidifying process for the same airstream before a combustion chamber of the Brayton cycle.        6. Management of water migration within the known system is realized but with poor efficiency and with a very complicated design.        7. The known system does not provide for efficient cleaning of stack gas from a turbine before its removal to the atmosphere.        
Using the known methods and designs, it is impossible to realize efficiently and economically all these useful actions simultaneously, and this decreases productivity of the known methods and systems for producing power.
Accordingly, it is desired to provide methods and systems for producing power resolving one or more of the aforementioned drawbacks.